Positioning a subscriber terminal, that is, determining its geographical location, is an important feature in cellular radio networks. In the United States, FCC (Federal Communication Commission) requires that it must be possible to determine the position of any subscriber terminal initiating an emergency call with a resolution of 50 meters at the finest. The location information can also be utilised for commercial means, for example, for determining various tariff zones or implementing a navigation service for guiding the user. Location services (LCS) have thus far been developed primarily for applications in circuit-switched cellular radio networks such as the GSM system (Global System for Mobile Communications).
There are various methods for implementing the location service. On the coarsest level, the position of the subscriber terminal can be determined by the identity of the cell serving it. This information is not very accurate, as the cell diameter can be tens of kilometers.
A more accurate result can be obtained by utilising timing information of the radio connection, for example Timing Advance (TA), as supplementary information. In the GSM system, TA will indicate the position of the subscriber terminal at a resolution of approximately 550 meters. The problem is that if the cell uses an omnidirectional antenna, the position of the subscriber terminal can only be determined as being on a certain perimeter drawn around the base station. A base station with three separate sectors will make the situation slightly better, but even in this case, the position of the subscriber terminal can only be determined as being inside a sector 120 degrees wide and 550 meters deep at a certain distance from the base station.
Even these inaccurate methods are adequate for some applications, for example, determination of tariff zones. In addition, more accurate methods have been developed. Usually, these methods are based on several different base stations making measurements of signals transmitted by the subscriber terminal, an example being the TOA method (Time of Arrival).
The subscriber terminal can also make measurements of the signals transmitted by a number of base stations, an example of such a method being the E-OTD method (Enhanced Observed Time Difference). In synchronised networks, the subscriber terminal can measure the interrelations between the moments of reception of signals from various base stations. In non-synchronised networks, the signals sent by the base stations are also received at a Location Measurement Unit (LMU), sited at a known fixed point. The location of the subscriber terminal will be determined from the geometrical components calculated from the time delays.
Another method for determining the position is to use a GPS (Global Positioning System) receiver fitted into the subscriber terminal. The GPS receiver will receive signals from at least four satellites orbiting the Earth; from these signals, it is possible to calculate/determine the latitude, longitude and altitude of the subscriber terminal. The subscriber terminal can either determine its position independently, or it can be assisted. The network component of the radio system can send the subscriber terminal an assisting message to make positioning quicker and thus reduce the power consumption of the subscriber terminal. The assisting message may contain the time of day, a list of visible satellites, the Doppler phase of the satellite signal, and the search window for the code phase. The subscriber terminal may send the received information to the network component that will then perform the actual calculation/determination of position.
In this application, the network component of a radio system means the fixed part of the radio system, that is, either the whole system excluding the subscriber terminal, or a specified element of the network (that is, not all network functions require all elements of the network, and thus the word ‘network’ can also refer to an operation performed by a single element of the network). Therefore, the network component comprises network elements communicating with each other in various ways.